Z, the number of either plagioclase, clinopyroxene, or opaque oxide crystals per unit volume, varies across the widths of 10 'flash-injected' diabase dikes in a manner which is well described by the equationwhere y is the distance from the contact. This relationship can be shown to be a consequence of the interaction of crystal growth and nucleation during the cooling of the dikes ifThe value of the exponent n in [i] is a function both of m in [ii] and the dominant growth-controlling mechanism during the main nucleation period. The n values of the minerals in the dikes examined cluster around −0.9, −1.5, and −2.2. On the basis of these data it appears that m is approximately constant and equal to 3.0 in basaltic melts. On the other hand, it appears that the dominant growth-controlling mechanism during nucleation is dependent on the exact composition of the melt. For example, the clinopyroxenes in the quartz and olivine tholeiitic diabases studied, differ significantly both in their crystal shape and n values. Departures from [i] are found within a few centimetres of the contact where, because of the rapid cooling, the assumption of constant m in [ii] is not valid.